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<div class="doctools">
<h1 class="doctools_title">pt::ast(n) 1.1 tcllib &quot;Parser Tools&quot;</h1>
<div id="name" class="doctools_section"><h2><a name="name">Name</a></h2>
<p>pt::ast - Abstract Syntax Tree Serialization</p>
</div>
<div id="toc" class="doctools_section"><h2><a name="toc">Table Of Contents</a></h2>
<ul class="doctools_toc">
<li class="doctools_section"><a href="#toc">Table Of Contents</a></li>
<li class="doctools_section"><a href="#synopsis">Synopsis</a></li>
<li class="doctools_section"><a href="#section1">Description</a></li>
<li class="doctools_section"><a href="#section2">API</a></li>
<li class="doctools_section"><a href="#section3">AST serialization format</a>
<ul>
<li class="doctools_subsection"><a href="#subsection1">Example</a></li>
</ul>
</li>
<li class="doctools_section"><a href="#section4">Bugs, Ideas, Feedback</a></li>
<li class="doctools_section"><a href="#keywords">Keywords</a></li>
<li class="doctools_section"><a href="#category">Category</a></li>
<li class="doctools_section"><a href="#copyright">Copyright</a></li>
</ul>
</div>
<div id="synopsis" class="doctools_section"><h2><a name="synopsis">Synopsis</a></h2>
<div class="doctools_synopsis">
<ul class="doctools_requirements">
<li>package require <b class="pkgname">Tcl 8.5</b></li>
<li>package require <b class="pkgname">pt::ast <span class="opt">?1.1?</span></b></li>
</ul>
<ul class="doctools_syntax">
<li><a href="#1"><b class="cmd">::pt::ast</b> <b class="method">verify</b> <i class="arg">serial</i> <span class="opt">?<i class="arg">canonvar</i>?</span></a></li>
<li><a href="#2"><b class="cmd">::pt::ast</b> <b class="method">verify-as-canonical</b> <i class="arg">serial</i></a></li>
<li><a href="#3"><b class="cmd">::pt::ast</b> <b class="method">canonicalize</b> <i class="arg">serial</i></a></li>
<li><a href="#4"><b class="cmd">::pt::ast</b> <b class="method">print</b> <i class="arg">serial</i></a></li>
<li><a href="#5"><b class="cmd">::pt::ast</b> <b class="method">bottomup</b> <i class="arg">cmdprefix</i> <i class="arg">ast</i></a></li>
<li><a href="#6"><b class="cmd">cmdprefix</b> <i class="arg">ast</i></a></li>
<li><a href="#7"><b class="cmd">::pt::ast</b> <b class="method">topdown</b> <i class="arg">cmdprefix</i> <i class="arg">pe</i></a></li>
<li><a href="#8"><b class="cmd">::pt::ast</b> <b class="method">equal</b> <i class="arg">seriala</i> <i class="arg">serialb</i></a></li>
<li><a href="#9"><b class="cmd">::pt::ast</b> <b class="method">new0</b> <i class="arg">s</i> <i class="arg">loc</i> <span class="opt">?<i class="arg">child</i>...?</span></a></li>
<li><a href="#10"><b class="cmd">::pt::ast</b> <b class="method">new</b> <i class="arg">s</i> <i class="arg">start</i> <i class="arg">end</i> <span class="opt">?<i class="arg">child</i>...?</span></a></li>
</ul>
</div>
</div>
<div id="section1" class="doctools_section"><h2><a name="section1">Description</a></h2>
<p>Are you lost ?
Do you have trouble understanding this document ?
In that case please read the overview provided by the
<i class="term"><a href="pt_introduction.html">Introduction to Parser Tools</a></i>. This document is the
entrypoint to the whole system the current package is a part of.</p>
<p>This package provides commands to work with the serializations of
abstract syntax trees as managed by the Parser Tools, and specified in
section <span class="sectref"><a href="#section3">AST serialization format</a></span>.</p>
<p>This is a supporting package in the Core Layer of Parser Tools.</p>
<p><img alt="arch_core_support" src="../../../../image/arch_core_support.png"></p>
</div>
<div id="section2" class="doctools_section"><h2><a name="section2">API</a></h2>
<dl class="doctools_definitions">
<dt><a name="1"><b class="cmd">::pt::ast</b> <b class="method">verify</b> <i class="arg">serial</i> <span class="opt">?<i class="arg">canonvar</i>?</span></a></dt>
<dd><p>This command verifies that the content of <i class="arg">serial</i> is a valid
serialization of an abstract syntax tree and will throw an error if
that is not the case. The result of the command is the empty string.</p>
<p>If the argument <i class="arg">canonvar</i> is specified it is interpreted as the
name of a variable in the calling context. This variable will be
written to if and only if <i class="arg">serial</i> is a valid regular
serialization. Its value will be a boolean, with <b class="const">True</b>
indicating that the serialization is not only valid, but also
<i class="term">canonical</i>. <b class="const">False</b> will be written for a valid, but
non-canonical serialization.</p>
<p>For the specification of serializations see the section
<span class="sectref"><a href="#section3">AST serialization format</a></span>.</p></dd>
<dt><a name="2"><b class="cmd">::pt::ast</b> <b class="method">verify-as-canonical</b> <i class="arg">serial</i></a></dt>
<dd><p>This command verifies that the content of <i class="arg">serial</i> is a valid
<i class="term">canonical</i> serialization of an abstract syntax tree and will
throw an error if that is not the case. The result of the command is
the empty string.</p>
<p>For the specification of canonical serializations see the section
<span class="sectref"><a href="#section3">AST serialization format</a></span>.</p></dd>
<dt><a name="3"><b class="cmd">::pt::ast</b> <b class="method">canonicalize</b> <i class="arg">serial</i></a></dt>
<dd><p>This command assumes that the content of <i class="arg">serial</i> is a valid
<i class="term">regular</i> serialization of an abstract syntax and will throw an
error if that is not the case.</p>
<p>It will then convert the input into the <i class="term">canonical</i> serialization
of the contained tree and return it as its result. If the input is
already canonical it will be returned unchanged.</p>
<p>For the specification of regular and canonical serializations see the
section <span class="sectref"><a href="#section3">AST serialization format</a></span>.</p></dd>
<dt><a name="4"><b class="cmd">::pt::ast</b> <b class="method">print</b> <i class="arg">serial</i></a></dt>
<dd><p>This command assumes that the argument <i class="arg">serial</i> contains a valid
serialization of an abstract syntax tree and returns a string
containing that tree in a human readable form.</p>
<p>The exact format of this form is not specified and cannot be relied on
for parsing or other machine-based activities.</p>
<p>For the specification of serializations see the section
<span class="sectref"><a href="#section3">AST serialization format</a></span>.</p></dd>
<dt><a name="5"><b class="cmd">::pt::ast</b> <b class="method">bottomup</b> <i class="arg">cmdprefix</i> <i class="arg">ast</i></a></dt>
<dd><p>This command walks the abstract syntax tree <i class="arg">ast</i> from the bottom
up to the root, invoking the command prefix <i class="arg">cmdprefix</i> for each
node. This implies that the children of a node N are handled before N.</p>
<p>The command prefix has the signature</p>
<dl class="doctools_definitions">
<dt><a name="6"><b class="cmd">cmdprefix</b> <i class="arg">ast</i></a></dt>
<dd><p>I.e. it is invoked with the ast node the walk is currently at.</p>
<p>The result returned by the command prefix replaces <i class="arg">ast</i> in the
node it was a child of, allowing transformations of the tree.</p>
<p>This also means that for all inner node the contents of the children
elements are the results of the command prefix invoked for the
children of this node.</p></dd>
</dl></dd>
<dt><a name="7"><b class="cmd">::pt::ast</b> <b class="method">topdown</b> <i class="arg">cmdprefix</i> <i class="arg">pe</i></a></dt>
<dd><p>This command walks the abstract syntax tree <i class="arg">ast</i> from the root
down to the leaves, invoking the command prefix <i class="arg">cmdprefix</i> for
each node. This implies that the children of a node N are handled
after N.</p>
<p>The command prefix has the same signature as for <b class="method">bottomup</b>,
see above.</p>
<p>The result returned by the command prefix is <em>ignored</em>.</p></dd>
<dt><a name="8"><b class="cmd">::pt::ast</b> <b class="method">equal</b> <i class="arg">seriala</i> <i class="arg">serialb</i></a></dt>
<dd><p>This command tests the two sbstract syntax trees <i class="arg">seriala</i> and
<i class="arg">serialb</i> for structural equality. The result of the command is a
boolean value. It will be set to <b class="const">true</b> if the trees are
identical, and <b class="const">false</b> otherwise.</p>
<p>String equality is usable only if we can assume that the two trees are
pure Tcl lists.</p></dd>
<dt><a name="9"><b class="cmd">::pt::ast</b> <b class="method">new0</b> <i class="arg">s</i> <i class="arg">loc</i> <span class="opt">?<i class="arg">child</i>...?</span></a></dt>
<dd><p>This command command constructs the ast for a nonterminal node
refering refering to the symbol <i class="arg">s</i> at position <i class="arg">loc</i> in the
input, and the set of child nodes <i class="arg">child</i> ..., from left
right. The latter may be empty. The constructed node is returned as
the result of the command. The end position is <i class="arg">loc</i>-1, i.e. one
character before the start. This type of node is possible for rules
containing optional parts.</p></dd>
<dt><a name="10"><b class="cmd">::pt::ast</b> <b class="method">new</b> <i class="arg">s</i> <i class="arg">start</i> <i class="arg">end</i> <span class="opt">?<i class="arg">child</i>...?</span></a></dt>
<dd><p>This command command constructs the ast for a nonterminal node
refering to the symbol <i class="arg">s</i> covering the range of positions
<i class="arg">start</i> to <i class="arg">end</i> in the input, and the set of child nodes
<i class="arg">child</i> ..., from left right. The latter may be empty. The
constructed node is returned as the result of the command.</p></dd>
</dl>
</div>
<div id="section3" class="doctools_section"><h2><a name="section3">AST serialization format</a></h2>
<p>Here we specify the format used by the Parser Tools to serialize
Abstract Syntax Trees (ASTs) as immutable values for transport,
comparison, etc.</p>
<p>Each node in an AST represents a nonterminal symbol of a grammar, and
the range of tokens/characters in the input covered by it. ASTs do not
contain terminal symbols, i.e. tokens/characters. These can be
recovered from the input given a symbol's location.</p>
<p>We distinguish between <i class="term">regular</i> and <i class="term">canonical</i>
serializations.
While a tree may have more than one regular serialization only exactly
one of them will be <i class="term">canonical</i>.</p>
<dl class="doctools_definitions">
<dt>Regular serialization</dt>
<dd><ol class="doctools_enumerated">
<li><p>The serialization of any AST is the serialization of its root node.</p></li>
<li><p>The serialization of any node is a Tcl list containing at least three
elements.</p>
<ol class="doctools_enumerated">
<li><p>The first element is the name of the nonterminal symbol stored in the
node.</p></li>
<li><p>The second and third element are the locations of the first and last
token in the token stream the node represents (covers).</p>
<ol class="doctools_enumerated">
<li><p>Locations are provided as non-negative integer offsets from the
beginning of the token stream, with the first token found in the
stream located at offset 0 (zero).</p></li>
<li><p>The end location has to be equal to or larger than the start location.</p></li>
</ol>
</li>
<li><p>All elements after the first three represent the children of the node,
which are themselves nodes. This means that the serializations of
nodes without children, i.e. leaf nodes, have exactly three elements.
The children are stored in the list with the leftmost child first, and
the rightmost child last.</p></li>
</ol>
</li>
</ol></dd>
<dt>Canonical serialization</dt>
<dd><p>The canonical serialization of an abstract syntax tree has the format
as specified in the previous item, and then additionally satisfies the
constraints below, which make it unique among all the possible
serializations of this tree.</p>
<ol class="doctools_enumerated">
<li><p>The string representation of the value is the canonical representation
of a pure Tcl list. I.e. it does not contain superfluous whitespace.</p></li>
</ol></dd>
</dl>
<div id="subsection1" class="doctools_subsection"><h3><a name="subsection1">Example</a></h3>
<p>Assuming the parsing expression grammar below</p>
<pre class="doctools_example">
PEG calculator (Expression)
    Digit      &lt;- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'       ;
    Sign       &lt;- '-' / '+'                                     ;
    Number     &lt;- Sign? Digit+                                  ;
    Expression &lt;- Term (AddOp Term)*                            ;
    MulOp      &lt;- '*' / '/'                                     ;
    Term       &lt;- Factor (MulOp Factor)*                        ;
    AddOp      &lt;- '+'/'-'                                       ;
    Factor     &lt;- '(' Expression ')' / Number                   ;
END;
</pre>
<p>and the input string</p>
<pre class="doctools_example"> 120+5 </pre>
<p>then a parser should deliver the abstract syntax tree below (except for whitespace)</p>
<pre class="doctools_example">
set ast {Expression 0 4
    {Factor 0 4
        {Term 0 2
            {Number 0 2
                {Digit 0 0}
                {Digit 1 1}
                {Digit 2 2}
            }
        }
        {AddOp 3 3}
        {Term 4 4
            {Number 4 4
                {Digit 4 4}
            }
        }
    }
}
</pre>
<p>Or, more graphical</p>
<p><img alt="expr_ast" src="../../../../image/expr_ast.png"></p>
</div>
</div>
<div id="section4" class="doctools_section"><h2><a name="section4">Bugs, Ideas, Feedback</a></h2>
<p>This document, and the package it describes, will undoubtedly contain
bugs and other problems.
Please report such in the category <em>pt</em> of the
<a href="http://core.tcl.tk/tcllib/reportlist">Tcllib Trackers</a>.
Please also report any ideas for enhancements you may have for either
package and/or documentation.</p>
<p>When proposing code changes, please provide <em>unified diffs</em>,
i.e the output of <b class="const">diff -u</b>.</p>
<p>Note further that <em>attachments</em> are strongly preferred over
inlined patches. Attachments can be made by going to the <b class="const">Edit</b>
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar.</p>
</div>
<div id="keywords" class="doctools_section"><h2><a name="keywords">Keywords</a></h2>
<p><a href="../../../../index.html#ebnf">EBNF</a>, <a href="../../../../index.html#ll_k_">LL(k)</a>, <a href="../../../../index.html#peg">PEG</a>, <a href="../../../../index.html#tdpl">TDPL</a>, <a href="../../../../index.html#context_free_languages">context-free languages</a>, <a href="../../../../index.html#expression">expression</a>, <a href="../../../../index.html#grammar">grammar</a>, <a href="../../../../index.html#matching">matching</a>, <a href="../../../../index.html#parser">parser</a>, <a href="../../../../index.html#parsing_expression">parsing expression</a>, <a href="../../../../index.html#parsing_expression_grammar">parsing expression grammar</a>, <a href="../../../../index.html#push_down_automaton">push down automaton</a>, <a href="../../../../index.html#recursive_descent">recursive descent</a>, <a href="../../../../index.html#state">state</a>, <a href="../../../../index.html#top_down_parsing_languages">top-down parsing languages</a>, <a href="../../../../index.html#transducer">transducer</a></p>
</div>
<div id="category" class="doctools_section"><h2><a name="category">Category</a></h2>
<p>Parsing and Grammars</p>
</div>
<div id="copyright" class="doctools_section"><h2><a name="copyright">Copyright</a></h2>
<p>Copyright &copy; 2009 Andreas Kupries &lt;andreas_kupries@users.sourceforge.net&gt;</p>
</div>
</div></body></html>
